See eg https://demo-anonymous.piwik.org/index.php?module=Widgetize&action=iframe&widget=1&moduleToWidgetize=CustomVariables&actionToWidgetize=getCustomVariables&idSite=1&period=day&date=2016-03-07&disableLink=1&widget=1
For example in this case "Custom Variable value not defined" shows only 11.3% because it's calculated in relation to all tracked Custom Variables in total. However, I would expect to see about 80%.
This way it would be for example easy to answer the question how many users with a tracked MySQL version use version "5.5.5". In the example it shows "1.4%" but it's actually about 25%.